The invention relates to polycarbonate resins and more particularly to polycarbonate resin molded articles. This invention further relates to polycarbonate resin molded articles comprising dyes, which may be photochromic, and/or metallic particles.
Polycarbonate is an amorphous, thermoplastic resin that has properties desirable for many articles. For example, glazings, lenses and automotive headlight lenses made of polycarbonate have high impact resistance and strength. Polycarbonate resin also can be highly transparent and has a desirably high refractive index. Furthermore, the thermal properties of polycarbonate resin makes the resin conducive to processing by conventional techniques, such as injection molding. However, there are also various shortcomings relating to polycarbonate articles. For example, no method presently exists for making high quality dyed or photochromic dyed polycarbonate articles such as glazings. Moreover, no method presently exists for making high quality polycarbonate articles that possess a metallic, or glitter appearance.
Two current methods of incorporating organic dyes into thermoplastic materials, such as polycarbonate, involve either inclusion of organic dye throughout the molded thermoplastic material or imbibition of dye into a surface of the thermoplastic material. Existing techniques, such as injection molding, for including organic dyes throughout thermoplastic materials, such as polycarbonate, typically do not yield satisfactory results. The unsatisfactory results occur for several reasons, including the relatively high temperatures required for satisfactory injection molding and the relatively high glass transition temperatures of polycarbonate. For example, photochromic dyes such as naphthopyrans, spironaphthopyrans, and spirooxazines that are co-melted with polycarbonate typically break down when exposed to the relatively high temperatures present during polymer melting. As another example, polycarbonate has a stiff molecular structure that is reflected by the relatively high glass transition temperature. Therefore, even in the absence of photochromic compound break down, the stiff molecular structure of polycarbonate substantially inhibits full activation of the photochromic dye, since the photochromic dye must go through a geometric transformation in the polycarbonate to activate.
Imbibition of dyes into surfaces of polycarbonate also yields unsatisfactory results, related to the relatively high glass transition temperatures of polycarbonate. It is thought that the stiff molecular structure prevents dye from penetrating the polycarbonate. Modification of the surface structure of polycarbonate resin by treatment with a solvent is said to improve imbibition of dyes into polycarbonate. U.S. Pat. No. 5,268,231 discloses that cyclohexanone is an effective solvent for modifying the polycarbonate surface structure to accept dyes. However, the method described leaves the surface of the polycarbonate with a rough, orange-peel type texture that is unacceptable for many purposes.
Approaches to manufacturing photochromic articles from thermoplastic resin materials and to avoid thermal degradation to the dye additive have also included molding of the articles by room temperature casting techniques; see for example the descriptions in U.S. Pat. Nos. 5,219,497 and 5,531,940. However, in many instances it is desirable to use thermal processing techniques.
The Japanese Patent Application No. 59-128192 filed Jun. 21, 1984 and laid open on Jan. 11, 1986 describes films with photochromic additives that are inserted into a mold following which the mold is filled (insert-injection molding). However, the films used (with photochromic properties) are prepared by imbibing, or coating a pre-formed film. The photochromic colorants are not homogeneously dispersed in the film and lack the quality of a homogeneous dispersion.
Approaches to manufacturing polycarbonate articles that have a metallic, or glitter appearance have thus far been largely unsuccessful because metallic additives cause degradation of polycarbonate at high temperatures. Moreover, relatively high loading of a metal is typically required to produce a metallic appearance, which exacerbates the degradation problem. It would be desirable to produce semi-transparent polycarbonate articles having a metallic appearance, but a relatively low metal loading, for applications such as sunglasses. It would also be desirable if such metallic polycarbonate articles were also photochromic.
The method of the present invention disperses homogeneously throughout a solvated polycarbonate resin, the dyes, which may be photochromic, and/or a metallic additive. The solvent mixture is then cast to form a film of polycarbonate having the dye and/or metallic additive homogeneously dispersed therein. The cast film can be placed in a mold against a mold wall and a substrate resin injected into the mold behind the insert. The article is thus molded without exposing the dye to high temperature for long periods of time, thus avoiding degradation of the dye.